Building A Passive Refrigerator

​The 8th grade physics class came to our site to build a small passive refrigerator. Since throughput (refer to articles on throughput) is front and center on any of the projects designed we wanted to access available free energy as much as possible thus in the winter months the refrigerator would use the cold air from the outside to keep our food cold. The students were all familiar with the basic idea on how the fridge operates; hot air rises and cold air sinks. What they needed was to have a fuller picture of the principles involved such as Newton’s law of gravity, density, and molecular energy in regards to temperature.

As the diagram demonstrates the refrigerator is simply a box with an insulated two inch plastic pipe (blue) that allows the cool air under the north side of the house to flow into the bottom of the box. Since cold air sinks, with only an incoming pipe the cold air will not flow into the refigerator. Another two inch pipe (red) enters the top of the box and leads to the outside through the roof. This pipe is not insulated and warms to the temperature that is inside the house. Since hot air rises the air in this pipe moves upward and pulls the cooler air from under the house and through the fridge.

​The days began in the same way as previous educational blocks. We started with Bothmer exercises and then went over the academic portion of the lessons inside the house (refer to Waldorf School – Eighth Grade of 2014 – Building a Hot Water Solar Panel) followed by lunch . After lunch we develop practical skills while building the refrigerator.

It was a sunny day so we had plenty of electricity from the photovoltaic cells to operate a jig saw to make the cuts for the box.

Some students worked on cutting the wood while others laid out the cuts. Notice the student in the back looking at his work and deciding what to do with the framing square in his hand.

All cuts had to be accurate. Since they were using recycled wood students had to account for the different thicknesses of the wood. Knowing how to read a tape measure increases the skill of using fractions in a real situation.

Learning how to use tools empowers students and gives them confidence to try new adventures.

We were lucky this week because they were preparing for an assembly and they had to practice a song.

The joy of singing is evident when watching this class.

At the end of each day we played Capture the Flag. The playing field is the woods with the road representing the boundary between the teams.

Playing in the woods requires an increase of focus, balance and agility. The terrain is rough with trees, rocks and hills adding a challenge to the game.

While students worked there were questions and discussion.

Team work was needed. Many times there were disagreements on how to do the work and when the disagreements occurred the students often worked to iron out their differences while other times they asked for advice.

One of the byproducts of the work is having students holding their focus. If they cut on the wrong side of the line, make a wrong measurement, or do not follow the line, the cut will not be accurate.

As a teacher it was gratifying to see students so focused on their work.

Again they listened to instruction and applied the suggestions.

​At the end of the week we moved inside to review what we did. For the summer months the refrigerator was to use excess electricity to power Peltier elements which make use of a bimetal in which a 12 volt DC current is passed creating a cold side and a hot side. These elements have no moving parts, last a long time, and are easy to install. There was a challenge in removing the heat from the hot side and the project needed further thought; perhaps it will become another project for another class. Since our house is powered by the sun and we have enough electricity to power a freezer, we will continue to freeze water and use the ice in the refrigerator in the summer months until the challenges can be solved.

​A passive refrigerator addresses the “Law of Diminishing Returns”. (refer to The Law of Diminishing Returns). When an object such as a refrigerator wears out it can take more money and energy to fix it rather than to just obtain a new one. Right now our country is facing the reality of fixing our aging infrastructure such as our roads, bridges, water and sewage pipes, and the electrical grid. We are realizing that the costs of the resources to accomplish this are rising. Because most household furniture has no moving parts nor is it subjected to weather, we are all familiar with tables, chairs and chests of drawers that are very old. When furniture does need repair usually a local person can be found to fix the antique or perhaps it can be fixed by the owner. However, when a conventional refrigerator becomes old and needs repair there is a point when the choice to buy a new one makes more sense. This passive refrigerator, with no moving parts or refrigerant, is more like a piece of furniture that can keep food cold for decades and decades without the need for major repairs. It can be built and maintained locally and blend in with the kitchen cabinets.